Semiconductor devices are typically classified as either volatile semiconductor devices, which require power to maintain storage of data, or non-volatile semiconductor devices, which can retain data even upon removal of a power source. An example non-volatile semiconductor device is a flash memory device, which generally may be classified as NOR or NAND flash memory devices. Such flash memory devices may stack cells, or layers, on top of each other taking the form of a 3D architecture. When faster program and erase speeds are desired, 3D NAND flash memory is typically utilized, in large part, due to its serialized structure whereby program and erase operations may be performed on entire strings of memory cells. In vertical NAND strings, each layer has a different diameter due to a non-normal etching angle.
With respect to 3D NAND flash memory associated with circular cells, differences in the diameter induce different program performance which conventionally results in a broader program threshold voltage (Vt) distribution. The larger the diameter of the cell, the longer the corresponding program time. In this regard, such large diameters present challenges, such as programmatic inefficiencies with respect to program time. Additionally, as a string will include cells having varying diameters, the programming time for the string may be difficult to control. Accordingly, there is a need in the art to improve the program threshold voltage distribution of 3D NAND devices, which in turn improves (e.g., decreases) program time.